The present invention relates to a feedback control method of controlling the rotational speed of an internal combustion engine.
There is known a method of controlling the rotational speed of an internal combustion engine in the idling condition or the decelerating condition, which involves controlling the flow rate of intake air sucked into the engine when the engine is in the idling condition or the decelerating condition, namely, when a throttle valve disposed in an intake passage of the engine is at the idling position. According to this conventional method, the flow rate of intake air is controlled by adjusting the sectional area of a flow passage or the opening time period of a flow passage, by means of a control valve disposed in a bypass passage which communicates the intake passage at a position located upstream of the throttle valve to the intake passage at a position located downstream of the throttle valve.
The control valve is adjusted in accordance with a feedback signal indicating the difference between the detected actual rotational speed of the engine and a desired rotational speed in the idling conditiion or the decelerating condition. This feedback control operation of the flow rate of intake air is carried out not only in the idling condition or the decelerating condition of the engine but, also, in the ordinary driving condition of the engine.
However, in the above-mentioned conventional method, since the desired valve of the rotational speed of the engine is always maintained at a constant value, a satisfactory feedback control operation for controlling the rotational speed of the engine cannot be expected. That is, when the actual rotational speed of the engine is abruptly decreased passing through the desired value of the rotational speed, the rotational speed of the engine overshoots below the desired value to a considerable extent. Particularly, in the case where the desired value of the rotational speed is very low, for example, lower than 700 rpm, an abrupt decrease of the rotational speed passing through the desired value of the rotational speed often causes the engine to stall. On the other hand, when the engine is rotated at or above the rotational speed of the desired value, for example at or above 700 rpm, since the rotational speed of the engine is forcebly controlled to decrease to the desired value, acceleration of the engine cannot be accomplished with good response characteristics, in other words, acceleration feeling of the engine is poor.